Hydraulic fracturing is well established in the oil industry. In conventional hydraulic fracturing as practiced by industry, the direction of fracture propagation is primarily controlled by the present orientation of the subsurface ("in-situ") stresses. These stresses are usually resolved into a maximum in-situ stress and a minimum in-situ stress. These two stresses are mutually perpendicular (usually in a horizontal plane) and are assumed to be acting uniformly on a subsurface formation at a distance greatly removed from the site of a hydraulic fracturing operation (i.e., these are "far-field" in-situ stresses). The direction that a hydraulic fracture will propagate from a wellbore into a subsurface formation is perpendicular to the least principal in-situ stress.
The direction of naturally occurring fractures, on the other hand, is dictated by the stresses which existed at the time when that fracture system was developed. As in the case of hydraulic fractures, these natural fractures form perpendicular to the least principal in-situ stress. Since most of these natural fractures in a given system are usually affected by the same in-situ stresses, they tend to be parallel to each other. Very often, the orientation of the in-situ stress system that existed when the natural fractures were formed coincides with the present-day in-situ stress system. This presents a problem when conventional hydraulic fracturing is employed.
When the two stress systems have the same orientation, any induced hydraulic fracture will tend to propagate parallel to the natural fractures. This results in only poor communication between the wellbore and the natural fracture system and does not provide for optimum drainage of reservoir hydrocarbons.
Therefore, what is needed is a method whereby the direction of hydraulic fracture propagation can be controlled so as to cut into a natural fracture system and link it to the wellbore in order to increase hydrocarbon productivity and cumulative recovery. This means that the in-situ stress field has to be altered locally in an appropriate manner.